Solder bumps provide electrical and/or mechanical connection between two or more substrates. Contained paste deposition is a method of depositing solder bumps in which solder paste occupies a containment region or cavity in a so-called containment structure, and where the solder paste containment assembly remains substantially unmoved through the reflow portion of the solder bump formation process. The process solder bump formation via contained paste deposition further includes dispensing or depositing solder paste into deposition regions (hereinafter also referred to as cavities) in a donor substrate. The donor substrate with the solder paste-filled cavities, may be flipped over, and is aligned and in contact with or in close proximity to a receiving substrate with patterns of solderable metallization corresponding to the cavities. The entire solder paste containment assembly, including the donor substrate, receiving substrate and solder paste is heated in a controlled manner, or reflowed. During the reflow stage of the process, the solder powder in the solder paste melts and, within each containment region on the donor substrate, coalesces into a single solder ball or bump per cavity. As the molten solder ball or bump coalesces, it pushes against the bottom and sides of the paste containment cavity until it reaches an equilibrium position.
Hard masks, typically made of silicon, ceramic or metal, are the most promising donor substrates because of their versatility and ability to withstand high reflow temperatures. Photoimageable polymer masks, with few exceptions, commence to burn around 250.degree. C. and are useful only for deposition of low temperature solders. Moreover, polymer film processing equipment is not a common fixture in semiconductor fabrication facilities. Finally, photoimageable polymer masks must be made thin (no more than about 0.5 mm thick) in order to be properly exposed and developed.
Hard masks can damage the surface of the substrate being bumped (the receiving substrate) especially if the receiving substrate is a semiconductor wafer or an integrated circuit (IC). Mechanical damage to the receiving substrate surface can occur any time that the hard mask (donor substrate) makes direct contact with the surface of the receiving substrate.
A solution to the problem of mechanical damage to the receiving substrate is a paste containment apparatus that does not require direct contact between the donor substrate and the surface of the receiving substrate. The donor substrate, typically a hard mask with paste containment cavities on its surface, is aligned face-to-face with and positioned in close proximity to the receiving substrate. One challenge with this approach is transferring the solder bumps across the gap between the donor substrate and the receiving substrate. Moreover, it is necessary that a sufficient solder volume have been deposited and that alignment between the two substrates be maintained.
Key factors in determining volume of the solder bump are the volumetric metal loading of the solder paste (limited by the random close packing of spherical solder particles), and the volume of the paste containment region or cavity. The volume of a solder bump formed by contained paste deposition tends to be no more than about half of the volume of the paste containment cavity in which the solder bump is formed. During reflow, a solder bump must protrude out of the containment cavity so that the bump reaches across or bridges the gap and makes contact with solderable metallization on the receiving substrate. If the solder bump, while molten, does not make contact with solderable metallization on the receiving substrate, it will not transfer but rather remain in the cavity, thereby resulting in a defect on the receiving substrate. Deep paste containment cavities may provide adequate solder volume, but may not force the solder bump to protrude out of the cavity. Shallow paste containment cavities may force the bump to protrude, but may not provide adequate solder volume. Large, shallow cavities also may permit the solder bump to move around in the cavity, possibly out of alignment with the solderable metallization on the receiving substrate, such misalignment resulting in defects on the receiving substrate.
What is needed is a solder paste containment apparatus that provides that the reflowed solder bump protrude above the surface plane of the donor substrate so as to enable proximity transfer of the solder bumps from a donor to a receiving substrate. What is further needed is a solder paste containment apparatus that provides for deposition of adequate solder paste volume on the donor substrate, and, in turn, adequate solder bump volume resulting therefrom. Also needed is a solder paste containment apparatus that provides for alignment between the donor and receiving substrate such that transfer of the solder bumps from the donor to the receiving substrate is facilitated.